282 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 3 8 



that a warm period preceded the cold one. At the moment such 

 conclusions may be speculative, but they point the way to promising 

 investigations. 



Many years ago it was deemed probable that the deep-water sedi- 

 ments were more highly radioactive than terrigenous deposits, 

 although there was some doubt regarding the early determinations of 

 radium contents by Joly and by Pettersson in the samples they 

 examined, collected by the Challenger, Albatross, and Princess Alice II. 

 More modern methods adopted by C. S. Piggot in the analysis of 

 samples of globigerina ooze, red clay, and blue mud collected by the 

 Carnegie have shown that the earlier conclusions were well founded. 

 Only 28 sediments, obtained from stations scattered throughout the 

 Atlantic Ocean, were tested by Piggot — a small enough number to 

 represent such a vast area. With a few exceptions the specimens 

 proved to have an extraordinarily high concentration of radium as 

 compared with that of continental rocks, such as ancient sediments 

 and granite, and even more so as compared with basalt. The general 

 average of Piggot's analyses shows 6.52 g.XlO -12 radium per gram of 

 sediment; for comparison, an average for granite might be assessed 

 at 2.5X10 -12 and for basalt at 0.8X10 -12 , although many granites 

 give a figure more like that of basalt. Exceptionally high contents 

 noted by Piggot in sediments from the deepest parts of the oceans, 

 such as red clay, reached 21.40X10" 12 and 16.72 X10" 12 . 



What is the source of this radium and what is the geophysical 

 significance of the relatively high content of the coating of the ocean 

 floor? To elucidate the first problem, Piggot turned to minerals 

 believed to be in process of formation in the depths, and chose some 

 of the manganese nodules which reveal growth banding with trapped 

 clay layers between the coats of manganese oxide. Uranium was 

 found to be fairly evenly distributed throughout a nodule, and the 

 radium-content was high, although it varied in the different constitu- 

 ents of the nodules. The problem is therefore concerned rather with 

 the parent uranium than with radium itself, and it has been suggested 

 that the concentration is brought about by the numerous minute living 

 organisms in the sea which extract, more or less selectively, the salts 

 of uranium and radium from the sea-water, and incorporate them in 

 their skeletons. When they die their remains take the radioactive 

 material to the bottom with them. Piggot disagrees with this view, 

 and also with that of Pettersson, who ascribed the high concentration 

 to submarine vulcanism. The uranium in sea water must come 

 ultimately from igneous rocks which are broken down on land. It is, 

 of course, in solution in sea water, and if we are to judge by the radium 

 content, it is in much the same proportions there as in ordinary rocks. 

 Near the shore the radium content of the sediments only approximates 

 to that of the ordinary rocks of the continent ; thus the enrichment in 



